I am trying to understand if and how the following scenario is possible.

Discount rules are applied:

Category Price Rule: e.g. Get 25% OFF all Category X products.

Shopping Cart Price Rule: e.g. Get 10% OFF Your First Order.

Lets say a customer purchases a product from Category X, they should get 25% OFF.

They also purchase a product from Category Y (which has no discounts applied). They want to use the 10% OFF coupon since it's their first order.

When they apply the coupon, they get 10% OFF the a product from Category Y, but they also get 35% OFF on the Category X product (25% + 10% stacked).

How can one limit the 10% OFF coupon being stacked and applied to the already discounted products?

I have read other solutions where people suggest to replace the Category Price Rule with a Shopping Cart Rule to achieve the same effect, but this means that products will not show the WAS/NOW strikethrough price on the category or product pages.

Preventing Shopping Cart Price Rules (coupons) from being applied when there is a Category Price Rule active is also not ideal, customers want to use their first order discount coupon!